1. Field of the Invention
The present invention relates generally to a high capacity media handling device and, more particularly, to a method for dynamically lifting an elevator platform of a media input tray in a media handling device during an ongoing media process.
2. Description of the Related Art
Demand in the market for high capacity media handling devices is increasing due to flexibility of these devices. Different media can be selected by customers for use with these devices. Greater volumes of media can be loaded into these devices. These devices need a media input tray that can efficiently feed a high volume of media. The media input tray utilizes a pick mechanism that feeds media a sheet at a time to a media process, such as printing, copying and the like, and an elevator that lifts a large stack of media sheets so as to place the top of the stack at a pick position relative to the pick mechanism.
The pick position may be at any of a plurality of levels that intersect an inclined surface on a restraint dam of the input tray between the upper and lower ends of the dam. The pick mechanism is able to feed media a sheet at a time most reliably when the top of the stack is at a pick position. Thus, the top of the stack should be maintained at a pick position regardless of how much media is loaded on a platform of the elevator as long as the loaded media does not exceed its specified capacity.
When the stack of media sheets is lifted by the elevator after being loaded onto the elevator platform, there is a maximum upper limit that the top of the stack can reach; it is called the media home position. This limit is the highest pick position. The input tray has a first sensor, such as a photo-interrupter or other electro-mechanical switch, to detect the media home position. When the media sheets are picked one at a time from the stack to supply a media process, such as printing, the level of the top of the stack decreases and potentially could go below the lowest level of the pick position. The main function of the elevator is to return the top of the stack to the media home position before it goes below the lower limit of the pick position which is the same as the lower end of the inclined surface of the dam.
Timing for lifting the stack by the elevator is also important since the stack must not be lifted when the pick mechanism is feeding sheets of media from the stack. Doing both at the same time could introduce adverse forces on the stack since the pick mechanism is pressing downward on the stack as the elevator is lifting the stack upward. This could cause media multi-feeds or damage to the media sheets.
The window for lifting the stack by the elevator to reach the media home position will be smaller if the throughput of the media process is faster. Since throughput is a function of inter-page gap, as provided in this relationship:throughput(pages/min.)=(process speed×60)/(page length+inter-page gap),where: throughput=page out per minute (PPM); process speed=linear sheet speed in the system (mm/s); page length=length of sheet being fed (mm); and inter-page gap=gap between leading sheet and trailing sheet (mm), having a longer inter-page gap will result in a slower throughput assuming that the process speed is made constant.
To achieve the desired inter-page gap during lifting the stack to the media home position, either the lifting must be made faster or the lifting distance or travel made shorter. When the input tray is ready for the next pick page command but the elevator is still lifting, the input tray will detect an error condition since there is a possibility of lifting the platform and picking the media at the same time.
The input tray typically employs one of two approaches to control the operation of the elevator. In the case of the first approach, the input tray uses four sensors. The first sensor, as already mentioned, senses the presence of the top of the stack at the media home position. A second sensor, the same type as the first one, is used to detect if the input tray is already empty. When the second sensor is triggered, there is no need to actuate the elevator to lift the platform since there is no more media sheets stacked on the platform. A third sensor, being the same type as the first two, is used to detect if the stack on the platform is already low. When the third sensor is triggered, the operation of the elevator remains the same.
A fourth sensor, the same type as first three, is used to detect the media level when the stack should be lifted by the elevator. Thus, the fourth sensor is at the elevator turn-on level. This fourth sensor is relatively close to the media home position and thus to the first sensor. Making the media home position and the elevator turn-on level farther apart will delay the next pick page sequence and, in turn, increase the inter-page gap (IPG), thus resulting in a lower throughput. When the elevator turn-on level is detected, by way of example, only approximately three sheets (nominal) of media are fed. The platform will then be lifted by the elevator until the media home position is reached. There are no increases in IPG and delays on the throughput using this first approach. Also, the elevator lifting speed is made constant since the level difference of the elevator turn-on and home position is not varying. However, because the media level difference of home position and elevator turn-on position is small, the leading edge of the media sheet enters via the dam on the same location; hence the rate of deterioration of a wear strip on the dam is high under this first approach.
In the case of the second approach, the input tray utilizes only three sensors. These sensors are the same as first, second and third sensors used in the first approach. The second approach does not use the fourth sensor used in the first approach, that is, to detect the media level when the stack should be lifted by the elevator. Instead, from the media home position, the number of sheets fed by the pick mechanism is counted until a preset maximum count is reached. No additional IPG or throughput delay is introduced. When the maximum count is reached by the input tray, the elevator will lift the platform until the first, or media home position, sensor is attained. However, even though the media level difference of the home position and its start to lift is relatively higher here, it still does not utilize the whole range of the dam for the pick position. The sheets counted before lifting are limited to a certain distance to make sure no delays are introduced in the IPG. Therefore, the speed is made faster to reach the media home position on time. Wear on the dam is still confined to an upper portion of the dam inclined surface.
A third approach to lifting the stack during printing or feeding sheets might be to just stop the media process and to continue feeding only once the media home position is reached. With this approach, the delay between sheets will depend on the distance for lifting the stack or by increasing the lifting speed. Maximizing the pick position would mean long travel for lifting the stack.
Thus, there is still a need for an innovation that will coordinate lifting of the media stack by the elevator with other operations so as to increase productivity without imposing any adverse impacts such as concentrated wear on the dam inclined surface.